1. Field of the Invention
The invention concerns a method for continuously producing panels of lignocellulose-containing particles, whereby the binder is continuously applied to the particles, which are continuously shaped to form a mat. The mat is continuously pre-compressed and at the same time continuously preheated by the effect of a high-frequency high-voltage energy field. The mat which is guided onto a planar surface, is compressed to form the panels under the effect of further heat. Furthermore the invention concerns a device for carrying out such a method with a gluing machine for continuously applying glue onto the particles, with a mat-forming machine for continuously forming the particles to a mat, a prepress for continuously pre-compressing the mat, with a high-frequency heating unit for continuously preheating of the mat through applying a high-frequency high-voltage field and with a heatable press, which consists of two sheets of metal, which presses the mat which was guided onto a planar surface to panels by applying further heat.
2. Description of the Related Art
The invention thus refers only to methods and devices, which include at a minimum the pre-press and the high-frequency device which allows work continuously, this minimal equipment does not include sawing devices. The invention later considers methods and devices which hot-presses the mat, which was guided onto one planar surface into panels. Thus the application of calander-presses, which can be used only for panels with limited thickness and special material is excluded. In contrast the invention is not limited to a special binder, the invention is also not limited to a defined particle size or composition of the lignocellulose-containing particles. This means it does not matter whether the binder is urea-formaldehyde resin or a binder free from formaldehyde. In the same way it does not matter, whether the manufactured panels are particle boards, medium density fibre boards (MDF) or oriented strand boards (OSB). However the invention has distinguished advantages when manufacturing certain type of boards. A method and a device of the sort described in the preceding are described in "Proceedings 27.sup.th International Particle board/- Composite Materials Symposium" W.S.U. 1993, page 55 to 66:
SUCCESS STORY. MODERN PARTICLEBOARD USING EASTERN HARDWOODS. There a device for the continuous manufacturing of particleboard is described which establishes the continuous preheating of the mat using a high-frequency heating device before the pre-press for mat compression. The high-frequency heating unit increases the mat temperature from room temperature by about 40.degree. C. It was reported there that by using high-frequency heating, the productivity of the line significantly increases because the pre-heated mat is pressed for a significantly shorter time in the finishing press. The heating-capacity of a high-frequency heating unit directly depends upon the field strength of the working alternating electrical field. For example, in order to reach the same heating-capacity when doubling the distance between the electrode platens the alternating voltage is to be doubled. Increasing the voltage increases the threat of high-voltage break downs which may lead to severe damaging of the high-frequency heating device. Additionally electric- magnetic impulses, which accompany break downs, may damage other electric or electronic installations. Finally, while producing panels, break downs may lead to ignition of the mat or to damaged areas on the finished boards.
Taking these problems into consideration, it is therefor necessary to locate a high frequency heating unit, for the continuous pre-heating of the mat, which has a working high-frequency alternating high voltage field, after the pre-press for continuous compressing of the mat, where the mat has only a reduced thickness. Therefor a significantly reduced distance between the electrodes of the high. frequency device is possible. In "Taschenbuch der Spanplattentechnik, Deppe/Emst, 3. Auflage", on page 175, such a high frequency-pre-press is mentioned together with a calander-line, where the mat is not conveyed onto one planar surface, but around a heating drum, while being heated. Calander presses are, as already mentioned, only suitable for producing thin panels. A production of OSB panels is impossible with calander-lines because of the spring-back properties of large two-dimensionally shaped wood particles. How a high-frequency pre-press for a calander-line should be built up is not shown in the immediate context nor in literature cited from Taschenbuch der Spanplattentechnik. One has, however, to proceed on the assumption, that when dealing with a known calander-line sequence, after a pre-press follows a high-frequency heating device. Essentially building in a high-frequency heating device in a calander-line is relatively unproblematic because the mat out of which thin boards are to be manufactured is itself thin, thus allowing for narrow distances between the electrodes of the high-frequency heating device.
High frequency heating units are also applied in lines for manufacturing of boards out of ligno-cellulose containing particles which do not work continuously. On the one side high frequency-heating of the mat is known for use with hot-pressing in a multi-daylight press. As multi-daylight presses with high frequency-heating are technically very high in expenditure and efficiency of high frequency-heating is limited the economy of such hot- presses is said to be poor.
Additionally, pre-compressors which do not work continuously which have a high frequency-heating for the mat during pre-compressing are known. There is a concern with one-daylight presses, which have a complicated technical construction, because the electrodes are spread over the whole length and width of the press for the homogeneous warming of the mat, whereby the length may lay in the range of some 20 meters. This means there is a problem because, for example, in the high frequency-heating device relatively high currents must flow, which are only manageable with high expenditures.